1. Field of the Invention
The present invention relates to an apparatus for forming a liquid crystal display, and more particularly to an apparatus for forming a combined substrate structure which comprises first and second substrates which are bonded to each other.
2. Description of the Related Art
The liquid crystal display includes two glass substrates and a liquid crystal filled into a gap between the glass substrates. The two glass substrates are, for example, a thin film transistor substrate and a color filter substrate. FIG. 1 is a schematic perspective view of a structure of a liquid crystal display. The liquid crystal display includes a thin film transistor substrate 2 and a color filter substrate 3.
The thin film transistor substrate 2 comprises a glass substrate having a dot-matrix array of plural pairs of a transparent electrode 35 and a thin film transistor 28. The thin film transistor 28 controls a voltage application to the paired transparent electrode 35 so as to control light-transmittivity of the liquid crystal. The color filter substrate 3 also has a dot-matrix array of color filters 36 of three primary colors, for example, red, green and blue. The color filters 36 make plural pairs with the transparent electrode 35.
The liquid crystal display also includes a back-light 30 for emitting a light and a first polarization filter 31 positioned between the back-light 30 and the thin film transistor substrate 2 for polarizing the emitted light as well as a second polarization filter 32 facing to the color filter substrate 3, so that the paired substrates 2 and 3 are interposed between the first and second polarization filters 31 and 32. The polarized light is transmitted through the thin film transistor substrate 2 and the transparent electrode 35. The polarized light is then incident into the liquid crystal between the thin film transistor substrate 2 and the color filter substrate 3.
Liquid crystal molecules 29 are ordered with a twist in a direction vertical to the surfaces of the thin film transistor substrate 2 and the color filter substrate 3. A twist angle of the liquid crystal molecules 29 depends on the applied electric field to the liquid crystal molecules 29. The electric field applied to the liquid crystal molecules 29 is proportional to the voltage applied between a pair of the transparent electrode 35 and the color filter 36. The color filter 36 is fixed at a reference potential of 0V. The control voltage is applied to the transparent electrode 35 under the control by the thin film transistor 28. Namely, the on-off switching operation of the thin film transistor 28 varies the potential of the transparent electrode 35, whereby the on-off switching operation of the thin film transistor 28 varies the voltage applied between a pair of the transparent electrode 35 and the color filter 36. Thus, the on-off switching operation of the thin film transistor 28 varies the twisted angle of the liquid crystal molecules 29.
The light is propagated along the twist of the liquid crystal molecules 29. An emitted light 33 from the back-light 30 is polarized by the first polarization filter 31. The polarized light is propagated through the liquid crystal along the twisted order of the liquid crystal molecules 29, wherein the polarization direction of the polarized light rotates in accordance with the twisted angle of the liquid crystal molecules 29. The polarized light is further transmitted through the color filter substrate 3 and the second polarization filter 32, whereby the liquid crystal display emits an output light 34. The second polarization filter 32 allows transmission of all of the light when the applied voltage to the transparent electrode 35 is 0V.
If a higher voltage is applied to the transparent electrode 35, then the ordered liquid crystal molecules 29 become oriented in the vertical direction to the surfaces of the color filter substrate 3, whereby the light is not twisted during the propagation through the liquid crystal, resulting in no transmission of the light through the second polarization filter 32.
As described above, the variation of the voltage applied to the transparent electrode 35 varies the intensity of the output light 34. Each dot is allocated with any one of the three primary colors. The lightness of each dot is controllable by controlling the voltage applied to the transparent electrode 35. Any images may be displayed by combination of dots, each of which has the pre-allocated hue and variable lightness.
The light transmittivity of the liquid crystal display is one of the important factors for the display quality. An inter-relation between the light transmittivity and an accuracy of the alignment between the thin film transistor substrate 2 and the color filter substrate 3 will be described with reference to FIGS. 2A and 2B. FIG. 2A is a fragmentary cross sectional elevation view of light transmissions through the liquid crystal display, wherein the thin film transistor substrate and the color filter substrate are aligned at a high accuracy. FIG. 2B is a fragmentary cross sectional elevation view of light transmissions through the liquid crystal display, wherein the thin film transistor substrate and the color filter substrate are miss-aligned at a low accuracy.
With reference to FIG. 2A, the thin film transistor substrate 2 and the color filter substrate 3 are aligned at a high accuracy, whereby the respective-paired transparent electrode 35 and color filter 36 are also aligned at the high accuracy. All of the polarized light as transmitted through the transparent electrode 35 are also transmittable through an entirety of the color filter 36. An effective aperture diameter “d1” is maximum. The transmittivity of the light through the liquid crystal display is high.
With reference to FIG. 2B, the thin film transistor substrate 2 and the color filter substrate 3 are miss-aligned at a low accuracy, whereby the respective-paired transparent electrode 35 and color filter 36 are also miss-aligned at the low accuracy. Only a part of the polarized light as transmitted through the transparent electrode 35 is transmittable through a part of the color filter 36. The effective aperture diameter “d2” is lower than “d1”. The transmittivity of the light through the liquid crystal display is low.
The light-transmittivity depends on the accuracy of the alignment between the thin film transistor substrate 2 and the color filter substrate 3. As the accuracy of the alignment between the thin film transistor substrate 2 and the color filter substrate 3 is high, then the light-transmittivity is high. As the accuracy of the alignment between the thin film transistor substrate 2 and the color filter substrate 3 is low, then the light-transmittivity is low.
If the accuracy of the alignment between the thin film transistor substrate 2 and the color filter substrate 3 is lower than what is shown in FIG. 2B, then a part of the light is leaked through an adjacent dot. This leakage of the light from the adjacent dot makes the display defect. In order to avoid this light leakage, the transparent electrode 35 and the color filter 36 are so designed as size-reductions by estimated errors in the alignment between the thin film transistor substrate 2 and the color filter substrate 3.
If the above issue of the low accuracy of the alignment between the thin film transistor substrate 2 and the color filter substrate 3 can be overcome, it is unnecessary that the transparent electrode 35 and the color filter 36 are so designed as size-reductions, whereby the effective aperture may be designed larger. The improvement in the accuracy of the alignment between the thin film transistor substrate 2 and the color filter substrate 3 and the design of the transparent electrode 35 and the color filter 36 without size-reductions results in a multiplier effect of great improvement in the display quality of the liquid crystal display.
As described above, the display quality of the liquid crystal display depends largely on the accuracy of the alignment between the thin film transistor substrate 2 and the color filter substrate 3.
The thin film transistor substrate 2 and the color filter substrate 3 are bonded by utilizing a thermosetting seal which is set upon a heat application. This process is so called as seal thermosetting.
FIG. 3 is a schematic view illustrative of the seal thermosetting apparatus for bonding the thin film transistor substrate and the color filter substrate. The seal thermosetting apparatus includes an entrance 11, a supporter 22, a heater 8, a heat-resistive HEPA filter 9 and a blower 10.
The combined substrate structure 1 comprising the thin film transistor substrate 2 and the color filter substrate 3 is supported by the supporter 22. The heater 8 generates a heat, whilst the blower 10 blows a hot air 7 through the heat-resistive HEPA filter 9. The hot air heats the combined substrate structure 1, whereby the seal sandwiched between the thin film transistor substrate 2 and the color filter substrate 3 is also heated to cause the thermosetting of the seal.
As shown in FIG. 3, the combined substrate structure 1 is heated, whilst opposite edges of the combined substrate structure 1 are supported by the supporter 22, so that the combined substrate structure 1 is slightly bent, and a center portion of the combined substrate structure 1 becomes lower in level than the opposite edges. The slightly bending of the combined substrate structure 1 causes a displacement or a miss-alignment between the thin film transistor substrate 2 and the color filter substrate 3. In this undesirable situation, the thermosetting of the seal between the displaced or miss-aligned substrates 2 and 3 appears. The product thus has the above-described miss-alignment between the thin film transistor substrate 2 and the color filter substrate 3, resulting in the deterioration in the display quality of the product.
If a temperature of the heat chamber is further increased, then a thermal strain appears on the supporter 22, whereby displacements of the supporting positions of the supporter 22 are caused. The planarity of the combined substrate structure 1 is also deteriorated by the displacements of the supporting positions of the supporter 22.
Japanese laid-open patent publication No. 10-104564 discloses an oven which is capable of holding a device horizontal at high accuracy. The supporter for supporting the device such as substrate is made of a material having a low thermal expansion coefficient, such as a crystal glass, wherein the supporter includes a horizontally supporter and poles. Even if the temperature of the oven is risen higher, then the strain of the supporter is suppressed small, whereby the device is well supported horizontally at the relatively high accuracy. As a result, the combined substrate structure has substantially no displacement or no miss-alignment between the thin film transistor substrate 2 and the color filter substrate 3.
The above conventional oven does not ensure the planarity of the device after the thermal expansion appears on the supporter. It is difficult to suppress the thermal expansion. It is also difficult to estimate the quantity of the strain. Even if the oven temperature becomes the ordinary temperature, it is uncertain whether the supporter once thermally expanded becomes the original shape. The frequency of the temperature rise and drop causes a permanent strain of the supporter. If the thermal strain is superimposed over the permanent strain, then the planarity of the device is deteriorated, whereby the device is defective. This results in the drop of the yield.
As the size of the glass substrates is increased, then the above problem becomes more serious because the supporter size is also proportional to the substrate size. The quantity of the displacement or the miss-alignment is also generally proportional to the size of the substrate or the size of the supporter, whereby the planarity of the device is deteriorated.
Japanese laid-open patent publication No. 9-281513 discloses a method, an apparatus and a system for assembling the combined substrate structure. Alignment marks are respectively put on respective substrates which make a pair. A displacement between the respective alignment marks is measured, so that the displacement or the miss-alignment between the respective substrates is compensated. The remaining displacement or miss-alignment in pitch between the alignment marks is further reduced by deformation of the substrates.
Even if the above described conventional method is applied to the seal thermosetting apparatus, it is difficult to obtain such desirable combined substrate structure at a high accuracy in alignment, because the conventional method does not consider the thermal expansion of the seal thermosetting apparatus and the combined substrate structure.
If the displacement or the miss-alignment between the combined substrates appears and a displacement of the alignment marks is caused, it may be proposed to deform the substrates for compensation to the displacement or the miss-alignment. If, however, at this time, the thermosetting reaction has already been started, it is difficult for the deformation of the combined substrate structure to compensate the miss-alignment or the displacement because the relative position between the combined substrates through the seal is fixed. Even if the deformation of the combined substrates is forced to align the alignment marks, and the deformed shape of the combined substrate structure becomes the original shape, then the displacement or the miss-alignment is caused again.
In the above circumstances, the development of a novel apparatus for forming a combined substrate structure free from the above problems is desirable.